In the production of electrical energy from nuclear reactor systems, the reactor provides heat for steam production and subsequent production of electricity. In the nuclear reactor, nuclear fuel rods are present which contain a nuclear fuel such as pellets of uranium dioxide or a mixture of uranium and plutonium dioxides. These fuel rods are metallic tubular shells, or cladding, which contain the fuel pellets and which must maintain their integrity so as to prevent any leakage into or out of the shell.
It is well-known that the incorporation, in various manners of a neutron absorber with nuclear fuel rods, which enables the use of excessive amounts of fuel in a reactor during the initial life of the fuel, can extend the life of the fuel rods. In some instances, the neutron absorber is mixed directly with the fuel and integrated therewith, while in other instances, a neutron absorber coating may be applied to the surface of fuel pellets, or discrete forms of a neutron absorber may be interspersed between conventional fuel pellets, or otherwise located within the cladding for the nuclear fuel. In U.S. Pat. No. 3,427,222, for example, a fuel rod is comprised of a tubular cladding that contains fuel pellets which have a fusion-bonded coating on the surface of each pellet, the coating comprised of a boron-containing material that functions as a neutron absorber.
It has also been proposed to provide cladding materials such as zirconium-based alloys that have various coatings or barrier means on the inside wall of the tubular cladding to protect the cladding from attack by constituents released from the nuclear fuel during operation of a reactor containing the fuel rod.
The use of a boron-containing compound in or on the cladding material has been proposed. U.S. Pat. No. 3,103,476 discloses the incorporation of a neutron absorber, such as boron, into the cladding of a nuclear fuel element. The boron is added to the cladding, which is preferably stainless steel, but may be zirconium or other material, in an amount of 200-1000 parts of natural boron per million parts of cladding material and homogeneously dispersed throughout the cladding. U.S. Pat. No. 3,625,821 describes a nuclear fuel element that has a zirconium or zircaloy cladding tube, with the inner surface of the tube coated with boron which is a neutron absorber. The boron is dispersed, as finely dispersed particles, in a matrix of nickel or other retaining metal.
Various methods have been proposed for formation of a coating on the inner surface of a fuel rod cladding. U.S. Pat. No. 3,274,066 teaches formation of a barrier material between fuel bodies and a graphite cladding shell by treating the graphite walls by exposure to a gaseous substance which can be deposited on the internal walls, and either simultaneously or subsequently reacted to chemically combine with the graphite to form a desired carbide. A vaporous silicon compound may be decomposed at a relatively low temperature in suitable atmosphere, such as hydrogen, to form a barrier layer of silicon carbide. In U.S. Pat. No. 4,229,260 a method of coating an oxygen-gettering material on the inner surface of a cladding is disclosed. The cladding is heated to 400-450.degree. C. while a volatile compound of the coating metal is heated to vaporize the compound and mixed with a heated inert gas. The mixture is then passed over the heated substrate whereby the vapor of the compound is decomposed and deposites the metal on the cladding. The coating is a layer of chromium preferably resulting from thermal decomposition of dicumene chromium.
Methods of vapor phase deposition of boron and borides onto metallic substrates are described in Chapter 11 of Vapor Deposition, Carroll F. Powell, Joseph H. Oxley and John M. Blocher, Jr.; John Wiley & Sons, Inc. (1966), pp. 343-352. The two methods described are (1) direct boride deposition from an atmosphere containing both boron and the metal component in the form of volatilized compounds; and (2) boronizing, or boriding, the surface layer of an object by heating it in an atmosphere of a volatile boron compound. The thermal decomposition of a boron hydride, such as diborane, or of an organoboron compound, such as trimethylboron are disclosed.
In the present process, the vapor phase deposition of a boron coating on the internal surface of a zirconium tube is effected at temperatures that do not affect the metallurgical structure of the tube for use in nuclear reactors, especially as a fuel rod cladding.